This invention relates to a method of and apparatus for fabricating a porous glass base material for an optical system such as optical fibers or rod lenses by means of a VAD (vapor-phase axial deposition) method.
When a porous glass base material for an optical system is fabricated by a VAD method, it is necessary to stabilize a gas stream in a reaction vessel in which an atmosphere for forming the base material is formed, and if the sufficient stability in the gas stream is not obtained, variations in the outer diameter and refractive index distribution of the porous glass base material increase.
To eliminate the variations, adequate means such as control of exhaust pressure of the reaction vessel, control of flow rate of the gas stream from the top to the interior of the reaction vessel or flow of the gas controlled under pressure have been conducted. Prior art inventions for this are disclosed, for example, in Japanese Patent Laid-open Nos. 69234/1981 and 135738/1982 official gazettes.
In FIGS. 3(a) and 3(b) showing the example of the conventional invention in Japanese Patent Laid-open No. 135738/1982, numeral 1 designates a reaction vessel having an exhaust port 2, numeral 3 designates a passage conductor inserted from the top to the interior of the vessel 1, numeral 4 designates a burner mounted at the end from the bottom to the interior of the vessel 1 for forming glass fine particles, numeral 5 designates a target, and numeral 6 designates a porous glass rod.
According to the conventional method exemplified in FIG. 3, a predetermined gas is flowed from the conduit 3 to the lower portion in case of forming the porous glass rod 6, the clearance in the conduit 3 is large at the initial stage of forming the glass rod 6 as shown in FIG. 3(a), and when the glass rod 6 is introduced into the conduit 3 as the rod 6 grows, the clearance decreases.
Therefore, as the glass rod 6 grows, the fluidity of the gas through the conduit 3 varies, and the directivity of the flame of the burner (the injecting direction of glass fine particles) in the vessel 1 also alters.
In the prior art invention disclosed in Japanese Patent Laid-open No. 69234/1981, though omitted for the description, similar phenomenon also takes place.
Since variation occurs in the directivity of the flame of the burner in the conventional method as described above, the method cannot effectively prevent the outer diameter or the refractive index distribution of the porous glass rod 6 from varying in case of fabricating the glass rod 6 having a desired length, and the control of the flow rate in the conduit 3 and the control of the pressure in the vessel 1 to be compensated are difficult.
On the other hand, if the inner diameter of the conduit 3 is increased to decrease the variation in the fluidity in the degree to be ignored, the outer diameter of the glass rod 6 is stabilized, but the flowing energy from the top of the vessel reduces in this case, the glass fine particles from the burner 4 behave at random in the vessel 1 and the stability of the refractive index distribution of the glass rod 6 is lost.